1. Field of the Invention
The invention relates to an improved high-pressure fuel pump for an internal combustion engine.
2. Description of the Prior Art
As general prior art in this field, German Patent Disclosures DE 102 36 314 A1 and DE 198 53 103 A1 are cited.
The invention is based especially on a so-called common rail system (CR system). The special feature of such CR systems is that the fuel quantity needed for injection must be brought to a variable pressure, dependent on the operating state of the engine at the time, by a high-pressure fuel pump. The high-pressure fuel pump is driven as a function of engine rpm, which can be done for example by means of the engine camshafts. The possible supply quantity of the high-pressure fuel pump is designed such that excess fuel, that is, more than the rail needs for the desired pressure buildup, can be pumped in every operating state.
From DE 198 53 103 A1, a fuel metering unit is known in which the flow quantity is controlled by a valve piston. For controlling the flow rate, the valve piston partially or entirely uncovers one or more control openings.
In this fuel metering unit, which is embodied as a slide valve, leakage, also called leak fuel, occurs in the overrunning mode of the engine, that is, when the high-pressure fuel pump is not supposed to pump any fuel, during the intake phase of the pump elements of the high-pressure fuel pump, and as a result in an unwanted way the high-pressure fuel pump aspirates fuel. This fuel is then put at high pressure in the ensuing working stroke and is pumped into the common rail. A pressure buildup, which is unwanted in the overrunning mode of the engine, occurs there and is reduced or limited by a pressure regulating or pressure limiting valve that is triggered by the control unit of the injection system. Since the fuel metering unit must be triggered by the control unit of the injection system, and in normal operation the regulation of the supply quantity of the high-pressure fuel pump is effected solely via the fuel metering unit, in this fuel injection system there are two structural groups, namely the pressure limiting valve and the fuel metering unit, that must be triggered by the control unit.